This invention relates to a method of recovering crude petroleum from subsurface earth formations, and more particularly, to a method of recovering crude petroleum from the subsurface by a steam and gas distillation drive process. The method may be used in primary recovery processes in highly viscous petroleum deposits as well as in secondary recovery processes where lighter crudes have already been recovered.
It has been known that residual oil can be produced from reservoirs by a combination of steam displacement and steam distillation. In a steam distillation process, hydrocarbons are more readily vaporized because of a lowering of their partial pressures in the presence of steam vapor. The light components are distilled from the residual oil and transported to the steam front where they condense and mix with the residual oil to form a bank of distillate and residual oil. As the steam zone advances, this distillate bank is displaced and redistilled to further increase oil recovery. With a steam distillation process it is possible to produce oil from a formation and to leave residual oil saturations below those obtainable by steam driving nondistillable oils.
A steam distillation recovery process is expected to recover more residual oil than a waterflood process because the steam distillation process (1) reduces oil viscosity, which improves oil mobility; (2) thermally expands the oil; (3) establishes gas drive from the steam vapor phase; and (4) distills the lighter oil components.
It is known to pump steam into a vertical borehole to produce a steam flood laterally into the formation in order to heat the oil in the formation to render it less viscous and to produce a driving force from the steam to move the oil to other recovery wells. It has been found in steam flood operations that the drive provided by the steam will collapse when the temperatures in the formation fall below the boiling point of water. In order to avoid the loss of these recovery mechanisms, inert or noncondensable gases have been added to the steam in order to enhance and maintain an oil-driving force within the formation.
As noted above, various attempts have been made to recover oil in a reservoir surrounding a vertical well by employing a mixture of steam and an inert or noncondensable gas. For example, in U.S. Pat. No. 3,908,762, a complex steam injection process is described which employs a mixture of steam and a noncondensable gas. In that patent the improvement is primarily based upon the disclosure that the noncondensable gases may include nitrogen, air, CO.sub.2, flue gas, exhaust gas, methane, natural gas, and ethane.
In U.S. Pat. No. 4,257,650, a process is described for horizontal well bores wherein a mixture of steam and noncondensable gas is injected into the formation from the horizontal well. The driving mechanism of the mixture in the formation may be selectively maintained or enhanced at the same time that the viscosity of the oil in the formation is being reduced due to the heat from the steam.
U.S. Pat. No. 4,086,964 describes the use of a foam-forming mixture of steam, noncondensable gas and surfactant injected into a steam channel in an oil reservoir in which stratification of the rock permeability is insufficient to confine steam within the permeable strata. The noncondensable gas added to the foam and steam is in very low concentration to stabilize the foam. The gas is included in fractions of a mole percent in the foam and the foam is intended to resist the flow of steam through the oil-depleted steam zone, thereby diverting the steam into undepleted zones.
These four representations of the prior art illustrate that it has been known to combine (a) heating of a reservoir with steam to increase the mobility of crude therein, (b) with the concept of heat distillation of the crude to develop a gas drive mechanism, (c) and the injection of a noncondensable gas with steam to further assist in the drive mechanism within the reservoir, (d) and the injection of foamable surfactant with steam and small amounts of noncondensable gas to improve the steam sweep efficiency.